Electro-optic image intensifier and method of making same



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1 P F G 3 A INVENTOR. 36 22 HAL LSOWERS x x DOMlN/CJ. RUGGIERI 32 JOHN I BALKWILL I G 3 W 3 MM ATTORNEYS United States Patent ice 3,458,744 ELECTRO-OPTIC IMAGE INTENSIFIER AND METHOD OF MAKING SAME Hal L. Sowers, San Carlos, Dominic J. Ruggieri, Fremont, and John T. Balkwill, Palo Alto, Calif., assignors to Optics Technology Inc., Palo Alto, Calif.

Filed June 2, 1966, Ser. No. 554,870 Int. Cl. H01j 39/00, 17/16, 61/30 U.S. Cl. 313-94 7 Claims ABSTRACT OF THE DISCLOSURE This invention relates to an image intensifier of the type having a photocathode that produces electrons in response to light excitation thereof and a fluorescent anode spaced from the photocathode which when excited by the electrons from the cathode fluoresces to produce a more intense image corresponding to the image impinging on the cathode. More particularly, the invention relates to an improved envelope structure for supporting the cathode and anode electrodes in operative spacial relationship and a method for fabricating such envelope structure.

The gain of image intensifiers, as well as the fidelity with which the input image is reproduced at the output, depend upon such characteristics as the spacing between the cathode and anode electrodes, the smoothness of the confronting faces of the electrodes, the degree of parallelism between the confronting electrode faces, and the like. An object of this invention is to provide a construction in which the foregoing characteristics are established at an optimum.

A more specific object is to provide an image intensifier structure adapted so that the electrode supporting transparent plates can be ground, lapped and polished after the plates are mounted to metallic supporting elements and prior to final assembly of the tube envelope. Achievement of this object is important because metal-to-glass sealing techniques typically involve application of extreme heat to accomplish the seal and such extreme heat is likely to distort the transparent plate. This object is achieved by at least partially imbedding a metallic member around the periphery of the glass plate so that the glass plate and the metallic member can be ground, lapped and polished at the same time to produce an optically fiat surface on which the electrode film is subsequently applied. An advantage flowing from attainment of the.

afore-stated object is that a substantially discontinuity free electrode face is provided so as virtually to eliminate arcing between the electrodes.

Another object, one made possible in part by the provision of optically smooth electrode faces, is the provision of an image intensifier tube wherein the confronting faces of the electrodes are mounted in very closely spaced relationship. Contributing to the attainment of this object is the provision in one of the metallic glass supporting members of a row of spacer protuberances which can be ground to precise parallelism with the electrode surface prior to final assembly of the tube envelope. The spacer protuberances are so formed and positioned that the di- 3,458,744 Patented July 29, 1969 mensional accuracy of the protuberances will not be adversely affected by the high temperatures necessary during welding employed in final fabrication of the envelope.

A feature and advantage of providing spacerprotuberances that are spaced apart from one another is that volumes on both sides of the protuberances communicate with one another so as to afford complete evacuation of envelope through only one opening in the envelope. Accordingly, the spacer protuberances provide precise alignment of the electrodes without interfering with fabrication of the tube envelope.

Another feature and advantage of providingan image intensifier tube in which the electrode faces are closely spaced is that for a given bias voltage, the gain, i.e., the intensity of the output signal as related to the intensity of the input signal, is improved.

Still another feature and advantage of the present invention is that the confronting electrode faces are substantially parallel with one another. The consequence of precise parallelism is that the fidelity of image amplification is made optimum throughout the entire electrode area.

Still another object is to provide an image intensifier tube in which the outer surface of one or both of the transparent electrode supporting plates is coplanar with or extends beyond the metallic supporting element on the tube. By achievement of this object the image intensifier tube can be employed in contact optics, for example in association with a video camera tube or cathode-ray tube. Contact optics avoids signal loss and/0r distortion caused by eventthe highest quality optical systems.

A further object of this invention, which contributes to the utility of the present invention in contact optics, is to provide an evacuating tubulation that extends radially of the tube and resides totally between the planes of the outer surfaces of the electrode supporting transparent plates. Known prior art structures of this type are so constructed that the evacuating tubulations extend beyond such planes thereby rendering impracticable employment of such prior art tubes in contact optics.

A still further object is to provide a method for joining transparent electrode supporting plates, which are typically formed of glass-like material, to metallic supporting members in a manner which does not interfere with the provision of an optically flat electrode supporting surface. Such method includes the'steps of providing a metallic supporting structure that has a rim generally parallel to the glass surface and a shape similar to the peripheral shape of the glass, heating at least the peripheral margin of the glass, partially imbedding the metallic member in the heat-softened glass, and simultaneously polishing the metallic member and the surface of the glass to form a continuous discontinuity-free surface. Avoidance of discontinuities in the surface ravoids arcing between the electrodes and also aifords an optically flat plane surface bounded by a conductive member on which the electrode films can be disposed. Electrical contact with the film is readily established through the metallic member.

These and other objects, features and advantages will be more apparent after referring to the following specification and accompanying drawing in which:

FIG. 1 is a side view in cross section of an image intensifier tube constructed according to the present invention;

FIG. 2 is a top view of the tube of FIG. 1, portions being broken away to reveal internal details;

FIG. 3 is a fragmentary view of a modification of the structure of FIG. 1 showing employment of the present invention in connection with optical fibers;-

FIG. 3A is a fragmentary view similar to FIG. 3 and showing a modification thereof; and

FIGS. 4A-4E depict in schematic form the steps of the method of the present invention.

Referring more particularly to the drawing, reference numeral 12 indicates a metallic ring which is joined in circumscribing relationship to an electrode supporting plate 14. The plate is formed of glass or like transparent, rigid, air-impervious material. An electrode film 16 which constitutes the anode of the tube is disposed on the inner surface of transparent plate 14. Electrode film 16 includes a layer of fluorescent material and contacts ring 12 at the peripheral margin of the transparent plate to afford electrical connection between external circuitry and the anode electrode. A second metallic ring 18 is provided in circumscribing joined relationship to an electrode supporting transparent plate 20 which carries on the inner surface thereof an electrode film 22 that forms a photocathode. An exemplary construction for the photocathode is a transparent film of conductive material such as stannous oxide or stannic oxide applied directly to transparent plate 20 upon which conductive film is coated a film of photoemissive material, such as antimony. As is conventional in devices of this type, electrode 16 is biased positively with respect to electrode 22, for example by a power source B, so that when light passing through plate 20 excites electrons from electrode 22, such electrons will be accelerated toward electrode 16 and will, on impinge ment on the latter electrode, form a visible image on the outer surface of transparent plate 14. For brevity in the following specification and claims, photoemissive electrode 22 and fluorescent electrode 16 will sometimes be referred to generically as electrodes.

Ring 12 includes an axially extending cylindric portion 24 at one axial extremity of which is provided a radially inwardly extending flange 26 and at the opposite axial extremity of which is provided a radially outwardly extending flange 28. Inwardly extending flange 26 supports transparent plate 14 and is joined thereto in accordance with the method to be described in more detail hereinafter.

Ring 18 includes an axially extending cylindric portion 30 at one axial extremity of which a radially inwardly extending flange 32 is provided and at the opposite axially extremity of which an outwardly radially extending flange 34 is provided. Inwardly extending flange 32 mounts transparent plate 20. As seen most clearly in FIG. 1, flange 34 includes a circular indentation for reinforcing the flange.

The side wall of the envelope is formed by a cylinder 36 formed of rigid, air-impervious, insulative material such as glass or ceramic which is joined at one axial extremity to flange 28 of ring 12 and at the other axial extremity supports ring 18. Cylinder 36 has extending radially therefrom one or more tubulations 38 for affording communication with the interior of the envelope during the final evacuation step in fabricating the tube. A centrally apertured disc 40 is provided for joining ring 18 to the end of cylinder 36 remote fromring 12. Disc 40 includes an inner radially extending margin 42 that defines a circular aperture 44 having a diameter approximately equal to the inside diameter of cylinder 36. Extending outwardly of margin 42 disc 40 defines a curved portion 46 radially outwardly of which is an outer margin 48 for effecting joinder of disc 40 to flange 34 of ring 18. Curved portion 46 imparts a degree of yieldability to disc 40 so as to avoid excessive stress on transparent plate 20 when disc 40 and ring 18 are conjoined by welding or like high temperature process.

Extending axially inwardly from flange 34 of ring 18 a plurality of spacer protuberances 50 are provided. The protuberances in the specific embodiment shown in the drawing, are formed in a circular row in approximate alignment with the annular surface of cylinder 36. Spacer protuberances 50 are spaced apart from one another so as to establish communication, during evacuation of the envelope, between tubulations 38 and the volume bounded by curved portion 46 of disc 40 and flange 34 of ring 18. Thus, it will be seen that when the tube is fabricated, it

will be efficiently evacuated in accordance with conventional techniques by attachment of a vacuum pump or the like to tubulations 48.

The tube envelope of the present invention is designed for efiicient and accurate fabrication. As a first step in such fabrication transparent plates 14 and 20 are joined in air-tight relation to rings 12 and 18, respectively.

Referring to FIG. 4, the method for joining transparent plate 20 to ring 18 is shown in detail, it being understood that the joinder of transparent plate 14 to ring 12 is effected in a substantially identical manner. Plate 20 has a larger outside diameter than the inside diameter of the opening bounded 'by flange 32 of ring 18. Flange 32 and the peripheral margin 'of plate 20 are heated, thereby softening the peripheral margin of the plate. The heating step is shown schematically in FIG. 4A. When desired temperatures have been attained, the plate and the ring are urged toward one another as shown at FIG. 4B and force is continued until flange 32 is at least partially im'bedded in the heat-softened peripheral margin of the plate. The plate and the ring are retained in such position until they cool to ambient temperatures. On cooling it will be found that the inner or lower surface of plate 20 that abuts the inner edge of flange 32 is depressed inwardly of the surface in a shape more or less similar to a meniscus as indicated at 52 in FIG. 4C. In the presence of high voltage the discontinuity at the joint between plate 20 and flange 32 would, if not eliminated, cause arcing. Such discontinuity is eliminated, as shown in FIG. 4D, by grinding, lapping and polishing the inner faces of the flange and the plate so that an optically fiat surface is provided which is bounded by conductive flange 32. Such surface is discontinuity free even at the joint between the flange and the transparent plate. The surface is indicated by reference numeral 54 in FIG. 4D.

Cylinder 36 is readied for joinder to flange 28 of ring 12 by cutting the cylinder to approximate length and then grinding the ends thereof to precise length and to precise parallelism between the respective ends. Ring 12, with transparent plate 14 joined thereto, and cylinder 36 are then placed in a. jig, not shown, to center the cylinder relative to the transparent plate. A glass or ceramic to metal seal between cylinder 36 and flange 28 is effected by RF heating which can be readily be applied so as to limit application of heat only to the area of contact between the cylinder and the flange. Margin 42 of disc 40 is joined to the opposite end of cylinder 36 by a similar technique. The use of RF heating in joining the metallic members to glass or ceramic cylinder 36 is important because precise parallelism between the electrodes depends on precise parallelism between the opposite ends of cylinder 36. Prior art flame sealing techniques are not suitable for joining the metallic members to cylinder 36 since such techniques tend to distort plate 14 and/or cylinder 36.

An electrode film 56 is next applied to surface 54, the composition of the film layer and the method of its application to the surface being well understood in the art. In the case of plate 20 which supports the cathode electrode film, layer 54 is a suitable photoemissive material whereas in the case of plate 14 which supports anode electrode the film layer or layers are selected for their fluorescent properties. Because of the fact that the smoothness of a thin film layer is proportional to the smoothness of the surface on which the layer is applied and because of the further fact that surface 54 is "optically smooth, the exposed surface of film layer 56 will be extremely smooth or flat. The grinding step, in addition to eliminating the discontinuity between flange 32 and plate 20, removes any irregularities on the surface of the plate that arise from warpage occurring during the heating step performed to join the plate to ring 18. As can be seen in FIG. 4E electrode layer 56 is applied beyond the outer surface of plates 14 and 20 and extends well onto the curved portion of respective rings 12 and 18. Accordlikelihood of arcing between the electrodes.

Before final assembly of ring 18 onto flange 48 of disc 40, the surfaces of spacer protuberances 50 are ground into coplanar relationship with the surface of electrode film 22. Flange 34 of ring 18 is then brought into contact wtih flange 48 of disc 40 and a metal-to-metal seal, for example by heliarc welding, is formed. The heat generated during performance of such step does not destroy the seal between cylinder 36 and flange 42 because of the relatively long heat conducting path afforded by curved portion 46. Such heat likewise does not adversely affect the seal between transparent plate 29 'andring 18 because of the relatively long heat conductive path afforded by flange 34, indentation 35, and cylindric portion 30 of ring 18. Moreover, the resiliency or flexibility afforded by curved portion 46 permits a degree of relative movement around the periphery of flange 48 so as to compensate for any stresses set up during the final welding procedure and thereby to avoid cracking transparent plate 20 or affecting adversely the glass-to-metal seals. Contacting of spacer protuberances 50 on the upper face of flange 42, which is joined to the annular face of cylinder 36, retains electrode film layer 22 in precise parallelism with electrode film layer 16. The tube is then subjected to the final steps including activation, heating and evacuation. The evacuation step is performed in a conventional way by attachment of a vacuum pump to tribulations 34; the spaces between protu-berances 50 permit the entire volume within the envelope to be evacuated.

Because the construction of the present invention avoids subjecting previously formed glass-to-metal seals to excessive temperatures during subsequent steps in fabrication, the construction is particularly suitable for employing optical fibers. With reference to' FIG. 3 there is shown a transparent plate 20 formed by a plurality of fused parallelly extending optical fibers rimming which is a sealing ring 58 of glass-like material configured in conventional form that functions to effect an air-tight seal with ring 18. The present structure lends itself to joinder of the fiber optic platesto the metallic rings by the use of frit material which affords an air-tight seal between the fiber optic plates and the metallic rings without necessity for subjecting the structure to temperatures sufficient to distort the fiber optic plates.

As described hereinabove in connection with FIG. 1 the inner surface of the" optical fibers is ground into coplanar relationship with the surface of flange 32 and electrode film layer 22 is applied as described above. The sub assembly constituted by ring 18 and optical fiber plate 20' is then fabricated into the remainder of the envelope as described previously without adversely affecting the smoothness of the electrode layer '22 or the seal between optical fiber plate 20' and the ring.

Referring to FIG. 3A, a transparent plate 14a formed of plural optical fibers is joined to metallic ring 12 by a sealing ring 58a of frit material, or the like. Plate 14a supports an anode electrode film 16 as in the structures described hereinabove so that the output image of the tube appears at the ends of the fibers remote from the anode film layer. The embodiment of FIG. 3A is particularly suitable for utilization in contact optical systems, because the outer ends of the fibers constituting plate 14a extend beyond the outer surface of ring 12.

The axial extent of cylindric portions 24 and 30 of rings 12 and 18 respectively can be made sufiiciently small with respect to the thickness of the transparent plates that the exterior surfaces of one or both plates can be formed outwardly of the planes of flanges 28 and 34 respectively. Consequently, image intensifier tubes constructed according to this invention are particularly suitable for contact optics. For example, a relatively low intensity image can be focused on cathode electrode 22 and the outer surface of plate 14, or plate 14a, can be disposed in direct contact with the surface of a camera tube, e.-g., an image orthicon. By employment of the image intensifier in such system the input signal to the camera is increased without distortion attending conventional optical systems that are interposed between the image intensifier and the camera tube input face. Correspondingly, the outer face of plate 20 can be placed in contact with a cathode ray tube face and the image derived at electrode 16 will be intensified without significant distortion.

Thus it will be seen that the present invention provides an image intensifier tube and a method for fabricating such tube that provides fora compact rugged envelope in which the electrodes are retained in precise spacial relationship. Because the intensifier tube of this invention is compact in the axial direction it is readily adaptable for employment in contact optics systems. Moreover, because the method of this invention affords sequential formation of glass-to-metal seals, tubes having optical fiber input and/or output windows can be successfully constructed.

While one embodiment of the present invention has been shown and described it will be obvious that other adaptations and modifications can be made without departing from the true spirit and scope of the invention.

What is claimed is:

1. An envelope for an image intensifier tube comprising: first and second conductive rings each having a central axially extending cylindric portion, an outwardly radially extending flange on one axial extremity of said cylindric portion, and an inwardly radially extending flange on the opposite axial extremity of said cylindric portion; first and second transparent rigid air impervious plates mounted on respective said rings and joined to respective said rings in air-tight relation to said inwardly radially extending flanges; a rigid air-impervious insulative cylinder attached to the outwardly extending flange of said first ring and extending generally parallel and concentric to the central cylindric portion of said first ring, said cylinder terminating remotely from said outwardly extending flange of said first ring in an annular surface parallel with said first transparent plate; a centrally apertured disc mounted on said cylinder, said disc including an inner margin defining a radially extending portion joined in airtight relation to said annular surface, a. curved portion extending outwardly of said inner portion and forming a radially yieldable 'wall portion, and a radially extending margin outwardly of said curved portion; said outer margin being joined in air-tight relation to said outwardly extending flange of said second ring; ,and means spacing last said flange in fixed relation of said inner margin of said disc to retain said second transparent plate in parallel relation to said first transparent plate.

2. The envelope of claim 1 including a 'tubulation extending radially outwardly from said iifs'ulative cylinder to affoi d communication with the interior of said tube during evacuation thereof.

3. The envelope of claifn 1 wherein the thickness of said first transparent plates exceeds the axial extent of the 5. The envelope of claim 1 wherein said flange spacing.

means comprises a plurality of spaced apart protuberances in the outwardly extending flange of said second ring, said protuberances being disposed in a circular pattern concentric with said secondring and congruent with the annular surface of said cylinder, said protuberances having mutually coplanar extremities remote from said flange,

said extremities lying in a plane parallel with the inner face of said second plate and contacting the surface of said disc opposite said annular surface.

6. A method for fabricating a photoelectric electrode comprising the steps of providing a transparent rigid member having a periphery, providing a conductive ring having an inner opening similar to and smaller than said periphery, said conductive ring having a reference plane approximately parallel to the opening, heating said ring and said transparent rigid member at the peripheral margin thereof, urging the ring and the transparent rigid member toward one another so that the ring is at least partially imbedded in a surface of the transparent rigid member, then finishing last said surface and said ring to form a discontinuity-free electrode plane in precise parallelism with the reference plane, said transparent rigid member forming the center of the electrode plane and said ring forming a peripheral margin of the electrode plane, and applying a photoelectric film to substantially all of said electrode plane.

7. A method of fabricating an image intensifier tube of the type having a first electrode carried on a surface of a first transparent rigid plate and a second electrode carried on a surface of a "second transparent rigid plate, one of the electrodes being a photoemissive cathode and the other electrode being a fluorescent anode, said method comprising the steps of providing a conductive ring having an axially extending cylindric portion and a radially extending flange integral therewith, mounting the first transparent rigid plate on said cylindric portion in air-tight relation thereto with the first electrode disposed axially opposite of the flange, forming a plurality of protuberances integral with and projecting from the flange in the direction of the first electrode, surfacing the extremities of the protuberances into mutual coplanar relation parallel to the first electrode, providing. a mounting structure for the second transparent rigid plate which mounting structure includes an axially extending cylindric wall portion that terminates remote from said second transparent rigid plate in an annulus, then surfacing the annulus to a plane parallel to the second electrode on said second transparent rigid member, and joining the cylindric wall portion of said mounting structure to the flange with the annulus and the protuberances in alignment to space said electrodes from one another in confronting parallel relation.

References Cited UNITED STATES PATENTS 2,601,208 6/1952 Kliever et a1 313-102 2,894,163 7/ 1959 Orthuber et al 313-94 X 2,975,015 3/1961 Davis 313- X 3,335,310 8/1967 Ney 31365 X 3,183,361 5/1965 Bronson et al 65--59 X JAMES W. LAWRENCE, Primary Examiner PALMER C. DEMEO, Assistant Examiner US. Cl. X.R. 

